The directions to the alleged brothel told the men that if they saw a house with green awnings, they'd gone too far. But some of them apparently misunderstood; would-be customers have shown up for years at the nearby house in Towson.

Despite neighbors' complaints, police say, Di Zhang, 42, continued to operate the brothel from a white Colonial-style suburban home on Joppa Road, advertising on websites until this month, when county police and federal agents moved in.

Neighbors said they weren't surprised to learn that Zhang, the operator of Jade Heart Health, had been charged with prostitution and human trafficking. They had long reported the operation to county authorities, and police had raided the business at least five times since 2007, according to charging documents.

Zhang is accused of forcing women to have sex with customers for money.

The business has remained at the house on a strip of boxy midcentury homes along the 1400 block of Joppa Road for years, frustrating regulators and police who say the case highlights the challenges in ferreting out and cracking down on brothels posing as legitimate businesses.

Prostitution and human-trafficking charges against Zhang in 2008 were dropped.

This time, federal Homeland Security agents were involved, raising the possibility of stiffer penalties that could close Zhang's business for good. According to state regulators, she hasn't had a massage license since 2004.

Zhang, who was released a day after the March 8 raid, has not entered a plea to the charges. She did not return telephone messages seeking comment. No one answered the door at her home, an attorney who represented her in previous cases did not respond to several phone messages and online court records do not show a current lawyer listed.

Police and organizations that work with victims say there are several businesses in the region like the one that Zhang is accused of running.

Such establishments often spring up along the York Road corridor, the Baltimore County Commission on Women reported last year. The road runs alongside Interstate 83, providing easy access to New York and Pennsylvania.

But as soon as authorities shut them down, they pop up elsewhere.

"You have to be able to react quickly," said Rod J. Rosenstein, the U.S. attorney for Maryland. And even then, he said, it's a challenge to get the victims to cooperate — they are often distrustful of law enforcement and sometimes unaware they are even victims.

Federal authorities don't generally take up prostitution or human-trafficking cases unless minors or foreigners are involved, or if it appears an operation spans state lines. Prosecutors declined to discuss details of the case.

The investigation began with a complaint from a neighbor, according to charging documents.

Homeland Security Special Agent David Snyder, the supervisor for investigations in the Baltimore region, said cases often start with such reports.

"We get leads all the time from neighbors," he said. "They see men coming out of the massage parlor. They are suspicious.

"The police are also great resources for us. They ride by these places. They get calls for service."

Gretchen Sarkin, president of the Loch Raven Village community association, said she had heard about the house: "Suspects walking back and forth, in and out of the back," she said.

Those who investigate human trafficking said no place is immune to the trade.

"It's pretty shocking to people in Baltimore County that there are lots of these types of spas," said Melissa Snow, the director of TurnAround, a center based in Towson that helps abused and battered women.

TurnAround is often called to assist female victims in human-trafficking cases in the county.

Investigators had pursued the case for days leading up to the raid, according to charging documents.

On March 4, detectives with the Baltimore County vice unit kept watch on the house. They observed a man entering the back door, they said in charging documents, and watched as he left less than an hour later.

An officer in a marked patrol car stopped the man, who said he had been greeted by a woman who called herself "Sylvia." He said he told her he wanted an hourlong massage, which she said would cost $70.

The man said he was taken to a room on the second floor of the house, where he was given a back massage. The woman also performed oral sex on the man, police say, and he gave her a $60 tip.

A few days later, detectives found an online advertisement: "new young girls just arrive in town Best services — Guaranteed!"

"They are so hot, beautiful face, killer body and smooth skin," the ad continued. "Hurry up don't miss this opportunity Very clean and No rush at all. Serious calls, no text messages, no email."

Investigators secured a room at the nearby Comfort Inn, and a detective called about the girls in the advertisement. Police say a woman on the phone told him a visit to the hotel would cost $220.

About an hour later, police say, a woman knocked on the door of the hotel room. After the detective showed her the cash, they say, the woman made a phone call and then took off her clothes.

After the woman agreed to have sex with the detective, police say, more officers entered the room and placed her under arrest.

On the night of March 8, about a dozen marked and unmarked police cars descended on the paved lot in front of the Joppa Road house. Officers from the county, joined by Homeland Security agents, used a battering ram to break down the green steel door and then searched the business.

Residents along Joppa Road say they're hopeful that the latest set of charges will keep the business from reopening.

Zhang pleaded not guilty to prostitution and human trafficking in 2008. Those charges were eventually dropped, but she was given probation before judgment on a charge of operating a massage business without a license.

Police raided Jade Heart Health in 2003 and closed down another business Zhang owned in Glen Burnie over zoning and fire code violations. She was charged with prostitution and massage license violations, but the disposition of that case was unclear.

James Vallone, executive director of the State Board of Chiropractic & Massage Therapy Examiners, said Zhang has not had a license since 2004. She unsuccessfully appealed the state's decision to revoke the permit.

Cpl. Cathleen Batton, a Police Department spokeswoman, said prostitution investigations can be difficult and time-consuming.

"We have to be able to substantiate the criminal activity," which can take time and repeated trips, and often involves uncooperative victims, she said.

While police address the prostitution supply, said TurnAround's Snow, the wider community should be addressing demand.

"We have to ask the bigger question of why" people solicit sex in Baltimore County, she said. "We need to do something to address the fact there are plenty of men willing to purchase sex from these women."